Wheelchair lifts of the type installed in the stairwell of transit vehicles, such as intra-city buses, are well known. One type of wheelchair lift, commonly referred to as a "step lift," is illustrated in U.S. Pat. No. 4,466,771 to Thorley et al. (the "'771 patent"). Another type of wheelchair lift, commonly referred to as a "platform lift," is illustrated in U.S. Pat. No. 4,058,228 to Hall (the "'228 patents").
Step lifts and platform lifts typically comprise a ramp or barrier plate which is pivotally mounted to the platform of the lift so as to be rotatable through an arc of about 180 degrees between a retracted position where the ramp confronts and extends parallel to the platform and an extended position where the ramp projects outwardly from the platform and is substantially coplanar with the platform. The ramp provides a planar surface between the ground and the platform when the latter is in the lower position, and acts as a vertically extending barrier (when in an intermediate position midway between the retracted and extended positions) for preventing a wheelchair from rolling off the platform when the platform is being moved between lower and upper positions.
Lifts of the type disclosed in the '771 patent typically include a linkage assembly for transmitting force from a hydraulic actuator to the ramp so as to cause the ramp to move between the retracted and extended positions. One portion of the linkage assembly is pivotally attached to the ramp, another portion of the linkage assembly is pivotally attached to the platform, and a third portion of the linkage assembly is attached to the hydraulic actuator. Generally, the linkage assembly is positioned in the center of the ramp and platform, i.e., midway between the left and right sides of the ramp and platform as viewed from the road looking into the stairwell in which the lift is positioned.
Linkage assemblies of the type used in the '771 patent project above the surface of the ramp and platform more than is desired, e.g., as much as one inch. As a consequence of the central placement of the linkage assembly and its projecting configuration, a hump is formed which tends to interfere with a wheelchair occupant's use of the lift. Governmental regulations regarding the design of wheelchair lifts now prohibit structure which projects above the upper surface of the ramp and platform more than 0.25 inch. Clearly, linkage assemblies of lifts of the type disclosed in the '771 patent do not comply with this regulation.
Furthermore, the design of linkage assemblies of the type used on the lift of the '771 patent typically include undesirably large openings or gaps between the various elements making up the linkage assembly and between the linkage assembly and the apertures in the ramp and platform in which the linkage assembly is received. These gaps open out to the upper surfaces of the platform and the ramp and are sufficiently large that the heel of a high-heeled shoe, an end of a cane, or a child's foot could possibly become lodged within a gap between linkage elements. To avoid such an occurrence, flexible covers have been used to block gaps between elements of linkage assemblies. Such covers add to the cost of the linkage assembly, can adversely affect the operation of the linkage assembly, and tend to require frequent maintenance. Absent the use of such covers, the gaps between elements of known linkage assemblies do not comply with current governmental regulations which permit gaps up to only 0.625 inch wide.
The construction of linkage assemblies of the type used in the '771 lift is such that the various components thereof are formed by various machining operations. These operations tend to be relatively time consuming, and hence expensive. Consequently, the total cost of a linkage assembly of the type disclosed in the '771 patent is typically more expensive than is desired.
Linkage assemblies of the type disclosed in the '771 patent are generally designed so that access to the attachment point of the end of the linkage assembly coupled to the ramp is via the bottom surface of the ramp. Because such bottom surface is typically covered with a tread that covers the attachment point, and the process for removing and reinstalling the tread is relatively time consuming, removal of the linkage assembly for maintenance or replacement tends to be more difficult and time consuming than is desired.
The design of linkage assemblies of the type disclosed in the '771 patent is such that the ramp is driven downwardly toward the extended position with a relatively large force. This force is sufficiently great that the person's feet are positioned underneath the ramp, the possibility exists that the person's feet could be crushed. To avoid the possibility of such an accident, a relief valve for limiting the presence of the hydraulic fluid supplied to the actuator is provided. Such a relief valve adds to the cost of the lift.
Yet another problem with linkage assemblies of the type disclosed in the '771 patent is that the various components thereof require frequent lubrication, thereby adding to the cost of maintaining the linkage assembly. Furthermore, the design of the components of such known linkage assemblies is such that the components tend to corrode, thereby adversely affecting the free operation of the portions of the lift coupled to the linkage assembly and increasing the stresses applied to various components of the linkage assembly and the lift.
Consequently, a need exists for a linkage assembly for a step lift of the type disclosed in the '771 patent, or for a platform lift of the type disclosed in the '228 patent, that does not project more than 0.25 inch above the upper surfaces of the platform and ramp and that does not include gaps of more than 0.625 inch. A need also exists for such a linkage assembly that can be serviced without removing the tread on the bottom surface of the ramp and that does not require periodic lubrication. A need further exists for a linkage assembly that is designed to drive the ramp downwardly toward the extended position with a force such that a person's feet inadvertently positioned in the path of travel of the ramp will not be crushed. A need additionally exists for a linkage assembly that is made from parts which can be fabricated quickly and inexpensively, preferably without the need for extensive machining operations.
Wheelchair lifts of the type disclosed in the '771 patent typically comprise a horizontal support for supporting the platform and vertically extending members attached, typically by welding, to the ends of the horizontal member. The vertically extending members are slidably received in vertically extending guides attached to the sidewalls of the stairwell in which the lift is installed. The vertically extending members coact with the guides to ensure the platform travels up and down along a predetermined path. Because space constraints permit the hydraulic actuator that raises and lower the step lift to be positioned only adjacent the rear side of the platform assembly, a chain drive assembly is provided for ensuring the front side of the platform assembly moves together with the rear side.
Unfortunately, under certain circumstances the guides do not extend parallel to one another and perpendicular to the surface of the platform. Such misalignment may occur as a consequence of improper installation of the guides, either originally or after maintenance, or can occur due to an accident of the vehicle in which the lift is installed. Furthermore, occasionally the chains of the chain drive assembly will become maladjusted.
Because the vertically extending members are rigidly attached to the horizontal member so as to extend perpendicular thereto, misalignment of the guides or maladjustment of the chains can cause the vertically extending members to bind as they travel up and down in the guides. Such binding can, in certain circumstances, cause the vertically extending members to break where they are attached to the horizontal member. Repair and/or replacement of the broken vertically extending members can be relatively time consuming because it requires the disassembly of a substantial portion of the lift. Space constraints prevent the addition of material to the junction of the vertical extensions and the horizontal support which could increase the strength of such junction sufficiently to prevent breakage.
Although arising in a different technological context, U.S. Pat. No. 4,579,500 to Robinson discloses a truck lift gate comprising a pair of vertical rails, a pair of elongate runners slidably received in the rails, and a platform, the outboard ends of which are pivotally attached to bottom portions of the runners. As a consequence of this construction, the platform is free to pivot slightly with respect to the runners.
The Robinson system differs significantly from lifts of the type disclosed in the '771 patent in that with the Robinson system forces for raising and lowering the platform are simultaneously applied to both ends of the platform. Furthermore, the Robinson system includes a pair of flat equalizer plates positioned adjacent the attachment point of the runners and the platform for causing the top surface of the platform to remain perpendicular to the long axes of the runners during vertical movement of the platform. Unfortunately, space constraints in the environment in which known step lifts are used prohibit the use of such equalizer plates, and prevent the application of the vertical drive force directly to both sides of the platform.
Thus, a need exists for a system for mounting the vertically extending members of a step lift to the horizontal member such that the vertically extending members do not bind or break during vertical movement in the guides in which they are received. A solution to this problem needs to be designed for incorporation into the chain drive system used for ensuring both sides of the step lift platform move together.